Use of Pt/CexZr1-xAl2O3 as Advanced Catalyst for Hydrogen Peroxide Thrusters L. Romeo 1, C. Genovese2, L. Torre3, A. Pasini4, A. Cervone5, L. d’Agostino6, G. Centi7 and S. Perathoner 8 ALTA S.p.A. - Via Gherardesca, 5 - 56121 Ospedaletto, Pisa, Italy and Dipartimento di Chimica Industriale ed Ingegneria dei Materiali – Salita Sperone, 31 – 98166, Messina, Italy
The capability of different Pt/Ce0.6Zr0.4/Al2O3 catalytic systems of effectively decomposing H2O2 has been studied in view of their application to monopropellant thrusters. BET surface area measurements, X-Ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM) have been used together with catalytic tests in order to evaluate the advantages of using CeO2-ZrO2 mixed oxide solid solution as an alternative to current three ways catalysts (TWCs). From the assessment of alternative solutions, a Pt/Ce0.6Zr0.4/Al2O3 catalyst suitable to effectively decompose H2O2 has been identified. SEM-EDX analyses ruled out the occurrence of phase segregation and selective deposition of Pt on Zr during the catalyst preparation. No changes in the crystalline arrangement of the catalyst samples after H2O2 decomposition have been detected by XRD measurements, except for a slight crystallization or grain size growth as a consequence of the high temperatures experienced during the reaction. The use of Pt/Ce0.6Zr0.4O2/Al2O3 catalysts for rocket-grade applications with much higher reaction rates than in automotive three-way converters has thus been successively demonstrated.
Nomenclature tmax s/t.n.
= time needed from the H2O2 decomposing solution to reach the peak temperature = seconds for test number
I. Introduction
T
HE first applications of hydrogen peroxide as propellant dates from 1938, when it was used in the assisted takeoff rocket system of the Heinkel He-176 aircraft and in the gas generator of the V2 rocket turbopump. Later H2O2 has been used in a number of applications culminating in the attitude control system of the manned Mercury spacecraft and in the primary propulsion of the Black Arrow launcher1. In order to take advantage of the H2O2 decomposition reaction in rocket propulsion applications, an effective, reliable and durable catalytic bed is required. It should provide fast and reproducible performance, be insensitive to poisoning by stabilizers and impurities contained in the propellant2, as well as capable to sustain the large number of thermal cycles imposed by typical mission profiles in small satellite applications. Traditionally pure silver and silver coated stainless steel grids3,4 have been the most used H2O2 catalysts in space propulsion applications, even if their use was associated with strong temperature limitations5. With the aim of developing more suitable substitutes, in recent years ceramic materials have been widely studied for the decomposition of hydrogen peroxide in gas generators or in monopropellant 1 2 3 4 5 6 7 8
Ph.D. Student, Dipartimento di Ingegneria Aerospaziale, Università di Pisa, AIAA Member;
[email protected] Post-Doc Researcher , Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Università di Messina;
[email protected] Project Manager, ALTA S.p.A., AIAA Member;
[email protected] Ph.D. Student, Dipartimento di Ingegneria Aerospaziale, Università di Pisa, AIAA Member;
[email protected] Project Manager, ALTA S.p.A., AIAA Member;
[email protected] Professor, Dipartimento di Ingegneria Aerospaziale, Università di Pisa, AIAA Member;
[email protected] Professor, Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Università di Messina;
[email protected] Professor, Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Università di Messina;
[email protected]
1 American Institute of Aeronautics and Astronautics
thrusters. In particular, both conventional alumina-based supports (extrudates, pellets, spheres)6,7 and honeycomb monolith carriers8 have been used for the preparation of catalysts. Under suitable conditions, the former supports can be directly impregnated using an active phase precursor, while the latter require surface impregnation of a thin and porous wash-coat layer, which is necessary to increase the relatively surface area of typical support materials, such as cordierite and mullite. According to the experience gained at Alta S.p.A. with catalytic beds for small rocket applications, alumina pellets with a medium content of the alpha-phase represents an effective solution for successfully withstanding the thermal shocking induced by the H2O2 decomposition reaction9. In order to optimize the properties of the alumina substrate and at the same time stabilize the active metal phase, Alta S.p.A., in collaboration with the Department of Industrial Chemistry and Material Engineering of the University of Messina, Italy, has developed a catalyst based on alumina spheres coated with a thin film of CexZr1-xO2, which is known to stabilize noble metal particles in oxidation reactions by strong metal-support interaction (SMSI)10. Platinum has been deposed on the mixed oxide support at concentrations of 6-10% by weight, in order to promote H2O2 decomposition. In this work we report the synthesis, morphological characterization and catalytic behavior of Pt/Ce0.6Zr0.4O2 in controlled hydrogen peroxide decomposition experiments at conditions representative of typical small rocket applications, emphasizing the relationships between catalyst structure and reactivity. It is to be stressed that this catalytic system, originally developed for automotive three-way converters, has been applied for the first time to H2O2 decomposition in small rocket engines, demonstrating high performance stability under extremely severe operational conditions.
II. Description of the Experiment A. Preparation of the Catalysts After a careful screening of different commercial Al2O3 supports, SASOL’s 0,6/170 spherical -Al2O3 (spheres of mean diameter of 0.6 mm with a surface area of 170 m2/g) has been chosen as the support for the Pt/Ce0.6Zr0.4O2 phase. The first step of the catalyst preparation consisted in the thermal treatment in air at 1100°C for 1 hr of the Al2O3 substrate in order to obtain the desired crystal phase with a measured BET surface area of about 74 m2/g (see Section III). The second step consisted in the preparation of a sol containing Ce/Zr in a 60-to-40 atomic ratio, starting from the correspondent nitrates (Aldrich) in the presence of citric acid and following a procedure already described in the literature11. The third step consisted in coating the Al2O3 spheres with the CeO2/ZrO2 sol precursor by wet impregnation and absorption. In particular, wet impregnation has been used for the CZ-4 sample to depose the Ceria-Zirconia film on the alumina substrate, while the other catalyst samples have been coated by means of the absorption technique. After impregnation, the samples have been calcined at 550°C for 6 hr. The forth step has consisted in the deposition of Pt by a modified ion exchange method using a Pt(NH3)4 (NO3)2 water solution for the CZ-4-CZ-9 samples and a H2PtCl6 solution in acetone for the CZ10-CZ11 sample. For the CZ-7 and CZ-8 samples the platinum precursor solution has been directly added to the Ce/Zr sol prior to carrying out the absorption step on the alumina substrate. The final Pt loading was 10% by weight for the CZ-7 and CZ-11 samples and 6% by weight for the remaining ones. Finally, the samples have been calcined at 500°C for 4 hr and reduced in H2 at 400°C for 4 hr. The main differences between samples CZ4 and CZ9 are mostly related to the increasing thickness of the CeO2/ZrO2 film. B. Characterization of the Catalysts 1. Catalytic Activity Tests A dedicated test bench has been realized by Alta for the comparative characterization of the activity and reaction rates of the catalyst formulations in controlled hydrogen peroxide decomposition experiments at atmospheric pressure. Figure 1 shows a schematic drawing of the test bench. It consists of a 250 ml reaction flask contained in a glass vessel with a volume of about 2 liters. The upper part of the vessel is closed by a sealing lid with the connection for the H2O2 supply funnel and the connection to the exhaust duct of the hot gas generated by the reaction. In the cover are also located two thermocouple taps, one for the measurement of the temperature of the liquid H2O2 an the other for measuring the temperature of the gas in the cylinder.
2 American Institute of Aeronautics and Astronautics
Figure 1. Scheme of the batch reactor
Before each test, a known mass of catalyst is put in the reaction flask. Next, a given quantity of hydrogen peroxide solution is added by opening the tap of the H2O2 tank. The decomposition reaction promoted by the catalyst generates a hot gas mixture at nearly atmospheric pressure, where molecular oxygen, steam and a small quantity of gaseous hydrogen peroxide are present. After leaving the vessel, these gases enter a heat exchanger, where their temperature is lowered by means of a cold coolant flow which passes through a coil pipe. The heat exchanger has been designed in order to condense most of water and hydrogen peroxide vapors in a liquid separator. As a consequence, the gas flow a the exit of the heat exchanger practically contains pure oxygen at known pressure and temperature conditions. In this way its flow rate, measured by means of a flowmeter, can be directly correlated to the rate of H2O2 decomposition. Temperature measurements are carried out by means of K-type thermocouples with a diameter of 1.5 mm and a length of 250 mm. The reduced order model used for the design of the test bench and the interpretation of the experimental results has been illustrated in a previous paper12.
2. BET Measurements The specific surface area (SBET) of the original Al2O3 supports and of the final catalysts has been measured by the BET method using N2 adsorption/desorption isotherms at 77 K using a Micromeretrics ASAP 2010 apparatus. Pore size and pore volume distributions have been obtained using the BJH method. 3. X-ray Diffractometry (XRD) X-ray diffraction measurements have been carried out using a Philips PW 1710 diffractometer with a CuK radiation. The XRD data were collected at 0.02° per step between 2 = 5° - 80°. 4. Scanning Electron Microscopy (SEM) EDX–SEM characterization of the catalyst samples has been carried out by means of a scanning electron microscope Jeol 5600LV. Elemental analysis has been carried out via energy dispersion analysis using an X-Ray analysis system EDX OXFORD, coupled with a scanning electron microscope.
III. Results A. Activity Tests For each prepared catalyst a sample of 0.85 gr, corresponding to a standard catalyst volume of 1 ml, has been selected. In order to measure the chemical activity, fifty consecutive drop tests have been carried out on each sample using the test bench previously described. In each test, the sample of catalyst spheres has been introduced in the reaction flask and 5 ml of 30% H2O2 hydrogen peroxide solution have been added. The temperature of the liquid mixture has then been continuously acquired until completion of the decomposition reaction. Finally, the initial conditions of the sample have been restored by removal of the decomposition Figure 2. Time history of the decomposing mixture products. The 30% H2O2 solution is produced by Riedel-de-Haen (Sigma-Aldrich) and exhibits a temperature during the 8th test on the CZ-9 catalyst. 3 American Institute of Aeronautics and Astronautics
particularly low content of impurities and stabilizers (PO43
